Optical signal processing techniques must handle optical signals having wide dynamic ranges, that is, signals having large differences in intensities between the strongest and weakest of such signals in the bandwidth of the input signals to be processed. The photodetectors, and other components, used in optical signal processing often have narrower dynamic ranges than the input signal to be processed.
Unless the dynamic range of the input signal is reduced before detection to within the dynamic range of the detector, information in the optical signal may be lost as a result of signals below the detector's threshold of sensitivity or signals which result in detector saturation. It is usually necessary in such applications to maintain the spatial frequency content of the input image while preventing detector saturation.
Post detection dynamic range problems have been handled by various conventional electronic dynamic range compression (DRC) techniques, but these techniques are not usable for pre-detection DRC, that is, for input signal compression before the optical signal is converted into an electrical signal by the detector.
One approach to pre-detection DRC for optical signals was proposed by the inventor hereof in a paper by Liu and Cheng: Appl. Opt., 27, 1006 (1988). In that paper, pre-detection optical DRC would be accomplished by means of beam coupling in infrared two-wave mixing using photorefractive crystals such as GaAs. Due to the response of GaAs, such devices can only be applied to near infrared input signals.
What is needed is a simpler, more convenient technique for optical pre-detection DRC, particularly for use in the spectral regions of visible signals.